Oxide semiconductor gas sensors can be miniaturized and integrated to desired sizes, are inexpensive, have high sensitivity and fast response, and can detect gas concentrations as electric signals using simple circuits. Due to these advantages, oxide semiconductor gas sensors are widely used in various applications, including detection of explosive gases, detection of exhaust gases from automobiles, measurement of drivers' blood alcohol levels, and detection of industrial gases. With the recent advances in high-tech industries and rapidly growing interest in human health and environmental pollution, there is a need for gas sensors for more precise detection of indoor/outdoor environmental gases, gas sensors for self-diagnosis of diseases, and gas sensors that can be used in high-performance artificial olfactory sensors mountable on mobile devices.
Some gases need to be detected. Among such gases, volatile organic compounds are known to be harmful to humans and are released from various sources, such as articles of furniture, solvents, and paints. Thus, it is very important to detect the concentrations of harmful volatile organic compounds in indoor environments. Representative substances harmful to humans in indoor/outdoor environments are volatile organic compounds, such as benzene, xylene, toluene, formaldehyde, and alcohol. Particularly, benzene, xylene, and toluene are aromatic hydrocarbons that have similar molecular structures. However, benzene, xylene, and toluene have different influences on humans. Benzene is known as a carcinogenic substance that can cause cancers, such as leukemia, whereas xylene and toluene were reported to cause various respiratory and nervous system diseases, such as ophthalmopathy and migraine.
Most oxide semiconductor gas sensors show comparable or similar sensitivities to the above five volatile organic compounds. However, the volatile organic compounds should be individually selectively sensed because they have different influences on humans as stated above. In the case where a sensor fails to separately sense the aromatic hydrocarbons and simply senses the total amount of the aromatic hydrocarbons, the problem arises in that it is impossible to appropriately decide how to respond to and solve individual sources of pollution. Alcohol gas occurs frequently and formaldehyde is also produced at a significantly high concentration during indoor activities such as cooking and drinking. For these reasons, gas sensors for detecting indoor aromatic environmental pollutants are required to have low cross-sensitivities to alcohol and formaldehyde. However, most oxide semiconductor gas sensors developed hitherto are highly sensitive to alcohol.